Cytotoxic T cells play a central role in immune-mediated control of cancers1-3, and monoclonal antibodies that target inhibitory receptors on T cells can induce significant clinical benefit in patients with advanced disease4-6. For survival, tumors have developed numerous immunosuppressive mechanisms to promote their own growth and to successfully evade the host immune system, effectively blocking the activity of T cells in the tumor microenvironment. This is a central issue in oncology because strong infiltration by CD8 T cells, which have cytotoxic function against tumor cells, is associated with a favorable prognosis in multiple types of human cancer1,3,8. This natural defense mechanism is severely blunted in the majority of patients by multiple inhibitory signals emanating from the tumor, its stroma, regulatory T cells and myeloid cell populations.9-11 Various molecular and cellular immunosuppressive mechanisms responsible for tumor evasion have been identified. Certain of these mechanisms target immune antitumor effector cells. However, many of the regulatory mechanisms that result in loss of T cell function within immunosuppressive tumors remain unknown. Improving on the limited success of cancer immunotherapy requires new approaches to inhibit immunosuppressive pathways initiated by tumor cells to evade the host immune system.